TU‐A‐BRC‐09: Feasibility Study of MRI in Assessing in Vivo Proton End‐Of‐Range for Liver Cancer

Purpose: To investigate the feasibility of using MRI to verify proton beam range in distal regions for liver tumor treatment. Methods: In the treatment of liver tumors with proton beams, the dose range uncertainty in the distal region can lead to reduced dose in tumor and/or increased dose in the surrounding normal tissue. Due to the increased extracellular fluid after radiation treatment, the irradiated areas in liver usually appear hypo‐intense on T1‐weighted MR images, and hyper‐intense on T2‐weighted MR images. This change of MR signal intensity (SI) allows for a quantitative verification of dose range in vivo. To achieve this goal, follow‐up T1/T2‐weighted MR images are firstly registered to the planning CT images. Then MR SI is correlated to the radiation dose at the superior/inferior penumbra dose fall‐ off, which includes two penumbrae in two proton beams. This SI‐dose correlation is finally employed on MR images to estimate the proton end‐of‐ range. This methodology is being evaluated on a 15‐patients database, which is being collected in our institute. Results: The preliminary results were based on three patients who received proton liver treatment. We observed correlations between MR SI and radiation proton dose in superior/inferior penumbra regions, with correlation coefficients (R2) of 0.86, 0.97, and 0.97, respectively. By applying the SI‐dose correlation to the distal region of proton beam, the mean distances from the MRI‐estimated dose range to the prescribed dose range were −0.4 mm, 2.6 mm, and 2.4 mm, respectively. Conclusions: The preliminary results demonstrate that the proton dose range can be verified in vivo to within 2.6 mm by follow‐up MR images after proton liver treatment. This IRB‐approved study is being extended to 15 patients with liver cancer treated by proton radiotherapy.